Tech Review - 2

Alas, another Sunday with coffee in tow.

In my reactor analysis course last semester, one group project focused on the effect the presence of U-236 would have on reusing uranium from spent nuclear fuel. It has long been known that U-236 introduces a long-lasting negative reactivity to the fuel. So-called penalty factors have been defined to quantify how many extra units of U-235 are needed to overcome the negative reactivity of one unit of U-236. These factors have been found to be roughly 0.30, or in other words, for every extra gram of U-236 produced, one must have present an additional 0.30 grams of U-235 to maintain the reactivity desired.

J. P. Renier et al. looked at this problem in depth, especially with respect to GNEP and waste disposal. The reuse scheme was to mix a sufficient amount of recovered uranium from an initial cycle with natural uranium to create fresh fuel for consecutive cycles. The first cycle was assigned 33 GWd/t burnup, typical of U.S. operations, and 55 GWd/t for subsequent cycles--why they focused on higher burnup for later cycles remains unclear to me. Perhaps it was merely assumed that future operations would employ such higher burnups.

Their conclusion states an asymptotic weight percentage of U-236 was found (~0.85%). The penalty factors for the cycles were roughly 0.25, which agrees pretty well with historical values.

Perhaps important to state is why U-236 would even be present. Uranium purification usually employs some form of centrifugal separation based on weight. For natural uranium, comprised only of U-235 and U-238, this works well, as the relative mass difference between the isotopes is large (enough). For U-235 and U-236 (produced during the cycle), this is not the case, and over half the U-236 remains with the re-enriched product.

The article is merely a transaction, so much detail is left out. What I'd like to know is how the data uncertainties look after propagation through 7 cycles. To me it seems perturbations in just about any factor could introduce significant changes in the U-236 concentrations (and hence the requisite U-235). Perhaps I'll solicit an opinion from the group who reviewed this work in class.